The division between President George Bush and Congress over the Iraq war hardened today as the House of Representatives again voted to withdraw most US combat troops from Iraq by April 1.

Although the vote was largely symbolic, the Democrats are planning a further series of votes in the House and the Senate over the next few weeks to try to tempt Republican dissidents.

The political manouevring in Washington came as US forces killed 13 people in a clash with Iraqi policemen in a Shia Muslim neighbourhood of Baghdad. The shootout at an Iraqi police checkpoint highlighted the extent of distrust the US has of the police force it helped establish.

In the Bush administration's report to Congress yesterday, on progress in Iraq, the state of the country's police force was judged to be unsatisfactory.

The US forces said today they had come under "heavy and accurate" fire from the Iraqi police checkpoint. Thirteen people were killed. The US troops called in a warplane that opened fire.

The US military, in a statement, said that an Iraqi police lieutenant had been detained on suspicion of planning roadside bomb and mortar attacks on US forces and having links with Iranians allegedly stirring up violence.

In a separate incident, a New York Times reporter, Khalid Hassan, 23, was shot dead on his way to his way to work in Baghdad. His death came less than 24 hours after two Reuters staff were killed in Baghdad.

Senior Republican senators who have broken ranks with Mr Bush over the last few weeks, including John Warner and Richard Lugar, are to publish alternative legislative proposals but they are unlikely to go far enough to satisfy the Democrats.

They are looking at legislation that would force Mr Bush to seek alternatives to his "surge" strategy. These include implementing a plan put forward by the Iraq Study Group, drawn up by senior political figures in Washington last December, at the request of Mr Bush, but subsequently rejected by him.

The Study Group proposed switching US troops from a combat role to a training one and to explore diplomatic negotiation with Iran's neighbours. But the Democrats want to go much further by setting a fixed timetable for withdrawal and threatening to cut funding for the war. In the end, the Democrats and Republicans may compromise on a bill that would switch US forces from a mainly combat role to a training role.

A serious vote in Congress may be delayed until September, when the US commander in Iraq, General David Petraeus, and the US ambassdor in Baghdad, Ryan Crocker, report back to Washington on any progress on the political and military fronts.

In the meantime, the Democrats will keep forcing votes on new bills to keep up pressure on their Republican counterparts. Many of the Republican senators who have withdrawn support for Mr Bush are facing re-election next year and fear that the Iraq war could cost them their seats.

Echoing Mr Bush, the US secretary of state, Condoleezza Rice, today urged dissident Republicans to delay action until September in order to allow time to "make a coherent judgement of where we are".

But White House aides are already regretting the focus on September, which in political terms is not far away. Congress breaks up in a few weeks for the summer recess and will not return until September.